Fundamentals of Voice over Frame Relay

Considering implementing Voice over Frame Relay? Brian Morgan, CCIE, provides a glimpse into this growing technology and dispels a few myths in the process.

Fundamentals of Voice over Frame Relay

by Brian Morgan - September 20, 2000

This article serves as botha basic and not-so-basic overview
of some of the technologies behind Voice over Frame Relay (a primer of sorts).
Implementations of this rapidly growing technology are numerous and continue
to push it to its limits. The transport of voice traffic over data networks
is an increasingly common implementation. It is important to gain an understanding
of the technology in general. This article will not cover VoFR in depth; however,
it does provide a basic understanding of the technology and assumes that the
reader is already familiar with Frame Relay usage and terminology.

Technology, and its evolution, is usually driven by one thingmoney. The
cost of telecommunications has always been an issue in every company. Whether
that cost relates to site connectivity or long distance calling charges, companies
are all looking for a way to cut costs.

What if it were possible to bypass some of the normal tolls involved in passing
phone calls from site to site within the company? There are already plans for
data connectivity (possibly already implemented). Why not pass voice traffic
across our data lines? That way, charges will not be assessed for long distance
calls.

This is the driving force behind VoFR technologies.

WARNING

Contrary to popular belief, you do not receive "free" phone calls
with any "voice over" technology. We've all heard the line, "Hey,
let's put voice on our data circuits. We're already paying for them, so the
voice calls will be free." This topic is discussed in the text of this
document. Get that thought out of your head now. Unfortunately, nothing is
free.

Frame Relay Basics

Voice over Frame Relay is defined by the Frame Relay Forum specification FRF.11.
This specification is discussed in its own section, "FRF.11
Voice over Frame Relay," a bit later. This section contains a very
brief review of basic Frame Relay operation.

Background Information

Frame Relay is a switching technology that was developed to facilitate point-to-point
as well as point-to-multipoint connectivity. Frame Relay was developed to support
high-speed data transfer over a telco network. Its purpose is to provide a way
of sending information across a WAN by dividing it into frames. Each frame has
an address that the network uses to determine the destination of the frame.
The frames travel through a series of switches within the Frame Relay network
and arrive at their destination.

Frame Relay networks are typically depicted as clouds, because the network
is not made up of physically connected endpoints. Instead, a logical path known
as a Virtual Circuit (VC) is defined within the network. Each of these virtual
circuits is differentiated from others using an identifier known as a DLCI.

DLCIs

Frame Relay is a data link layer technology, which makes use of unique connection
identifiers to differentiate conversations. These connection identifiers are
known as Data Link Connection Identifiers (DLCIs). DLCIs are usually statically
assigned through the configuration of Permanent Virtual Circuits (PVC). PVCs
are manually configured into each switch that the circuit must traverse. They
are similar to static routes manually configured into a router. Switched Virtual
Circuits (SVC) allow the dynamic assignment of DLCIs (SVCs are not common with
Frame Relay implementations and therefore won't be discussed further in this
article). A DLCI is locally significanta term that's been thrown
around with some frequency. Local significance means that this identifier has
meaning only on the present leg of a connection (i.e., a single leg of a connection
between a router and switch or between two switches). Consider Figure
1 as an example.

In the preceding figure, each leg of the connection has a DLCI that is meaningful
only for that leg of the transmission.

DLCIs are used in making switching decisions. When paired with an inbound or
outbound interface designation, the DLCI creates a unique piece of information
with which the switch can make a path decision. In Figure
1, for example, frames entering switch A through interface 1 having a DLCI
of 16 are switched to interface 2, and a new DLCI of 42 is assigned. The decision
is based on the switchs PVC definition.

Hopefully, everything we've covered to this point has been review material
because that's as deep as we go in this section. For more information on Frame
Relay in general, check out http://www.frforum.com.

Frame Relay Topologies

Frame Relay implementations have been deployed in a number of differing manners.
Overall, the ideas behind these implementations are similar. Frame Relay supports
full mesh, partial mesh, and hub & spoke topologies, as follows:

Full MeshThe full mesh topology is an "all-to-all"
implementation and tends to be the most robust. This topology can be considered
to have a form of redundancy built into it because each router has connections
to every other router (true redundancy involves multiple paths between each
router). Should one connection fail, connectivity to remote networks can still
be achieved via another router (as long as it was the link that failed, not
the router). This topology also results in the least delay for traffic moving
across the network. One disadvantage to the full mesh, however, is that it
is the most expensive topology to run.

Hub & SpokeThe hub & spoke topology consists of a
central site router with connections to all remote sites. This is the most
common type of topology implemented in Frame Relay installations. There are
usually very few, if any, redundant connections. Redundant connections are
typically provided by ISDN dial backup circuits.